Journal of Life Sciences 12 (2018) 30-46 D doi: 10.17265/1934-7391/2018.01.004 DAVID PUBLISHING The Origin of Spirula spirula (Linne, 1758) E. O. Heyfetz Nachman mi Braslaw 14/3, Yaffo 6808998, Israel Abstract: The article presents evidence regarding the origin of Spirula from spiral-shelled forms. The geological age of the species is revised from the Miocene to the Jurassic. The causes of the reorientation of the shell are elucidated. The mode of life of the mollusk is specified. The influence of the theory on facts perception is shown. Key words: Buoyancy, chambers, distribution and geological age, guard, keels, mode of life, origin, orientation of the shell, phragmacone, proostracum, Spirula L. 1. Introduction concern with the subject of work, were excluded. In spite of such limitation a number of facts, The spiral shell has been appeared many times unnoticed earlier, were found. It is natural that the during evolution of different shelled animals. There signs, seeming negligible ones, acquire importance by are many foraminifers, sessile bristleworm Spirorbis; another putting of the question, and in contrary: the gastropods; some bivalves (Exogirus, Gryphea, consideration of question from a “conventional” jewelbox clams, the rudist Toucasia, the extinct standpoint could just obstruct or even to distort Pliocenian cockle Prosodacna and so on). facts—that will be shown below. This moment is not In cephalopods external planispiral shell appears ever taken into account in the scientific periodic, independently in nautilids, ammonits and relying on authority of a specialist in such or another secondarily—in females of Argonauta. The results of branch. these processes are on hand: shell compaction as adaptation to the greater maneuverability (at the 2. Morphological expense of the speed). The other deal that on the way 2.1 Spirula and Familiar Forms to this solution was temporary rejection from active mode of life—pass from the straight shell to the bent Spirula is a relict cephalopod with internal spiral one and only afterwards—to the spiral one. shell. Currently belemnite-like Belemnoseina with Even more problematic appears the postulated bent chambered part of the shell (phragmocone) are tendency of internal shell spiralization, whose clear accepted for the ancestral group. example is spirula. The given article provides Such a standpoint could not be based empirically: arguments in favor of the contrary standpoint. In in contrast from belemnites, which are guiding fossils, contrast to lookthrough works, it represents no more belemnoseines are extremely rare, since their guard than spread out proof of the correspondent thesis. For consists not from the calcite, but from less stable this reason the author stands just on the certain aspects, aragonite [1]. The other deal intuitive ground: without pretending of the complete description of presence of specific endocochlear appendages in Spirula and familiar forms. Furthermore, in the course belemnoseines, permitting their approach with of arguments elaboration, those losing immediate ammonites, seems to be more important sign than spiral shape of shell, as was mentioned above, Corresponding author: E. O. Heyfetz, independent appearing independently in different mollusc groups researcher, research field: paleontologyand hydrobiology. and a number of other shelled animals. Correspondent The Origin of Spirula spirula (Linne, 1758) 31 Fig. 1 The origin of Spirula due to the current version. 32 The Origin of Spirula spirula (Linne, 1758) Fig. 2 The shells of the families Belopteridae (A) Belemnopsidae (B) and Spirulirostridae (C)—from the “fundamentals of paleontology. version was defined also by opposite spiral orientation utile volume of the body cavity, and, in contrast, the (see below). production of phragmocone leads to insignificant Meanwhile, the normal tendency in the evolution of increase of spending on shell building and to mollusc with the internal shell is its reduction up to its reduction of the utile volume of cavity, aggravated in complete disappearance. Apparent exceptions, like the bent and spiral form by the bending of shell, guard, increasing material spending, decrease the whose alternative is a simple increase of the working volume, i.e. the force of buoyancy of phragmocone diameter, like in belemnites. Meanwhile, cephalopods’ shell. the most bent forms of belemnoseines (e.g. From the phragmocone function as such it was Spirulirostra) possess a massive guard, reducing the assumed that the process of internal shell spiralization assumed adaptation to zero. was connected with development of passive buoyancy Furthermore, guards of belemnoseines are means. This is the classical version of the sepiae sufficiently more changeable than the phragmocones evolution [2] that obviously disregards with benthic (Fig. 2). mode of life of the majority of cuttlefishes. Since natural selection preserves only favorable In addition, from all the standpoints the most changes, the guard would reflect evolutional tendency efficient mean to increase buoyancy would be the in the first turn. Meanwhile, as was said above, in the reduction of guard. First and foremost, it would give boundaries of suborder of Belemnoseina the guard significant economy of material plus increase of the does not show inclination to reduction. In such a way, The Origin of Spirula spirula (Linne, 1758) 33 the conclusion about belemnoseines’ shell negatively: “…Among the most ancient coleoid there spiralization does not correspond both with relative are no one spirally coiled form. The impossibility of variability of its parts, and with needs of the molluscs. the coleoid origin from spirally coiled forms may be Both absence of any traces of guard and protoconch reinforced by logical arguments. in Spirula are not evident in favor of the accepted Proostracum—straight supportive structure, version. Questions about their traces were asked commeasurable by the length with the body—would repeatedly. Both Naef and Bruun emphasize that the not appear in animals with spirally coiled shell” [5]. juvenile Spirula completely lacks any traces of The spread objection as such is the evident about protoconch [2, 3]. Chun in after consideration of importance of the problem. The other deal, how lightful organ of Spirula initially accepted it for probative the arguments mentioned are. vestigial guard [4]. Later he established the real The internal shell bending in familiars of Spirula function of the organ that was interpreted before as an Bizikov considers as a juvenile adaptation to adheasing disc. Warnke and Boletzky (2009) accept planctonic mode of life [5]. for the remnant of guard a thin membrane, developing To this argument the mentioned contradiction on the ventral (i.e., anti-guard) side of initial chambers between the function of guard and phragmocone is of Spirula shell. applied. The robust capitulum, enveloping the first Bizikov elaborates the theoretical grounding of the phragmocone chambers from the ventral side (Fig. 3) current version. The author dwells on the question of is evident that the guard formation even preceded the coleoids’ origin from the spiral forms, and solves it phragmocone bending. Fig. 3 The comparative shell morphology of Belemnitida and Belemnoseina. 34 The Origin of Spirula spirula (Linne, 1758) Fig. 4 The pair of keels on the juvenile whorls of the shell of Spirula (drawn from the object). In the process of further shell growth the mantle Such a suggestion permits to look on the facts more sack, stretched on the phragmocone, lost the contact closely. Let us consider the shell, whose juvenile with its concave part and ceased to produce the guard bubble–like chambers differ so sharply from on the ventral side (Figs. 2 and 3)—in contrast to that barrel–like adult ones. On the internal (e.g. anti-guard) of belemnites. side of the juvenile whorl there is a pair of keels with The given peculiarity is preserved also in furrow between them (like runners). Externally it belemnoseines with straighten shell and their resembles siphuncle, nevertheless, considering descendants, true cuttlefishes, where bending of sections either samples with broken chambers it may phragmocone could not be obstacle for guards’ be easily ascertain that the given formations are development. This suggests that the straighten forms completely independent (Fig. 4). originated from the bent ones, and not on the contrary. In contrast to the guard of belemnoseines and Correspondingly, both absence of specific sepiids, that is no more than overflow, the keels are appendages, and the spiral shape of shell are clearly separated from the phragmocone. Appearing interpreted as the evidence of neothenic origin of the on the 2-nd chamber after protoconch they are mollusc [5]. smoothen towards 12-th, reaching 0.3 mm in height. The Origin of Spirula spirula (Linne, 1758) 35 Fig. 5 Cross-section of the juvenile whorl. 24—ventral rib. From 2. Due to their position and geometrically correct Naef—combination of membrane with keels. shape, they represent a kind of sheath that enveloped In such a way, the internal side of whorls, formerly external part of the previous whorl. contacting with previous ones, bears a whole number The given structure was not described in of evidences of its past significance, vividly showing details—that may be explained by its small size and that Spirula comes not from forms not with bent, but “obscure significance”. Naef restricts himself by with tightly spiral shell. mentioning of ventral rib of periostracum on the 2.2 The Ammonite Features of Spirula juvenile part of the shell [2], putting the drawing of first three chambers’ section, but with no detail A whole number of signs closing Spirula with description of the formation. ectocochlear molluscs are overlooked when it is Sometimes between the ventral sides of the considered from the standpoint of the current version. protoconch and the following chamber thin membrane Thus, the internal shell of Spirula is not inscribed in is preserved. It was described by Branco, 1880 and the contours of body, but is projected out on both Appelöf, 1893 and currently it was re-discovered by ventral and dorsal sides of the body.